Maleimides and polyimides have become important components in the plastics industry. Their application as plastics additives and in high performance composites enhances thermal and chemical stability, resistance to wear and impact, and ease of workability. Thus, the addition of 5%-10% N-phenylmaleimide (PMI) to ABS plastics increases their heat distortion temperatures to over 125.degree. C. Bismaleimides (BMI), such as 4,4'-bismaleimido-diphenylmethane (MDA-BMI), are monomers used in the production of high performance polyimide resins for application in structural materials in the aviation, electrical, electronics and aerospace industries. The improved properties they provide could benefit a wider range of applications which seek better performance from lighter weight materials. However, their high cost limits such opportunities.
Their high production cost is largely a result of the low yields of high purity product obtained, the complex operations involved and waste treatment, costs involved in the industrial processes used until now for the monomer manufacture. Their synthesis, by the condensation of amines with maleic anhydride in two successive steps (formation of amic-acids and imidization thereof, which, in the case of this invention, is performed by a thermal route), is deceptively simple. Yet, as reported by Breitigam and Stenzenberger (Soc. of Plas. Eng., Conf. on High Temperature Polymers, Oct. 2-4, 1989), the yields of polymer grade monomers of this family obtained are usually in the range of only 65%-75%. Isoimides, anilides, fumaramic acid derivatives and oligomeric by-products are obtained, among other impurities. In the case of BMI, any misdirected reaction with one of the amino groups is doubly felt, for it automatically renders the other amino-functionality unproductive as well. Not only are raw materials costs relevant to the process economics, but the manufacturing operations involved in complex purification steps, solvent recovery as well as difficult waste disposal problems, in some cases, of toxic materials, such as MDA, add to the manufacturing expense. For example, in the latter instance, MDA concentrations in the MDA-BMI product in the ppm range are required. Equally stringent restrictions are placed on the concentration of MDA in waste effluent. OSHA (Occupational Safety and Health Administration) suggests a 10 ppb exposure limit (100 ppb short term exposure) for this compound (Chemical Regulation Reporter, 0148-7973/89, May 19, 1989, page 197).
The thermal imidization of the amic-acids is acid catalyzed and is performed in an inert liquid medium, from which the water of condensation is removed by azeotropic distillation. The basic concept of this process is described in U.S. Pat. No. 2,462,835. However, the high temperatures required lead to unwanted isomerizations, condesations and polymerizations.
When the inert medium is a hydrocarbon or halocarbon, the insolubility of both the amic-acids and the maleimides leads to the precipitation of contaminated product. It has been reported (e.g. in JP 85,260,623; JP 86,106,554; JP 86,229,863; JP 236763/86; JP 159764/82) that the use of an apolar solvent in conjunction with an aprotic polar solvent leads to improved results. However, the reaction conditions applied, the methods used for product isolation and the quality of the maleimides produced are not satisfactory. For example, in JP 615976/82, using ethylene dichloride (EDC) and dimethylformamide (DMF) as the mixed solvent to produce MDA-BMI, the reaction time was 20 hours and the product was only 93% pure.
In JP 236763/86, the same material was produced in a mixture of toluene and DMF. Great care was exercised to control the distillation at a precise rate over a period of 15 hours. Yet the product purity was only 96%. Furthermore, it was recovered from the reaction mixture by distilling both solvents under reduced pressure and adding the remaining concentrate dropwise to an aqueous sodium carbonate solution. Both the p-toluenesulfonic acid (PTS) (used as a catalyst) and the DMF contained in the concentrate, dissolve in water and cannot be recovered in a practical way; this in addition to the ecological problem presented. This process is clearly inapplicable on an industrial scale.
JP 229863/86 describes a very similar process with the same unsatisfactory results.
JP 260,623/60 performed the dehydration for a shorter period of time and recovered the product by dropping the (sometimes concentrated) reaction mixture into water or by precipitation with excess methanol. Product of similar purity was obtained, but no practical solution is offered for the recovery of materials used. Neither is there any reference to the residual MDA level.
It should be noted that in all of these cited patents, amic-acid production is effected in the same mixed solvent mixture (but catalyst free) as that used in the second, imidization step. The importance of this feature will be seen below.